Aircraft control surfaces, for example flaps located on the trailing edge of a fixed wing, slats located on a leading edge of a fixed wing, spoiler panels, aileron surfaces, and the like, have traditionally been actuated by hydraulic actuation systems. More recently, electromechanical actuators (“EMAs”) have gained acceptance in the aviation industry for adjusting the position of control surfaces. EMAs are designed to sweep through a given stroke, linear or rotary, but must have definite points where the stroke must start and end. In practice, two sets of endpoints are defined: one set defines the electrical stroke and the other the mechanical stroke. In normal operation, EMAs are controlled by sophisticated integral or remote electronics over the electrical stroke. However, conditions may arise where an errant command results in the EMA being driven beyond the normal electrical stroke endpoint into a mechanical stroke endpoint. The endpoints that define the mechanical stroke are usually hard mechanical stops. Aircraft manufacturers require that the EMA contain the EMA stroke to prevent possible damage to the airframe or control surfaces. Because of usual space constraints in aircraft, extra room to include “soft” mechanically cushioned stops is not available. If an EMA is driven at sufficient rate into a mechanical end stop either during an in-flight event or as a result of a rigging error during assembly, significant damage usually occurs. After a “shearout” device is employed, and after an event, the EMA is rendered inoperative. A costly overhaul process is required to replace parts and return the unit to service.
It is known to use a rotary ball detent mechanism in an EMA system to limit the torque transmitted from an input gear to an output gear to a chosen maximum torque. The input and output gears are axially aligned on a drive shaft. After a stop is encountered, the rotary ball detent mechanism disconnects the driving inertia from the load path at levels that prevent damage. Conventional ball detent mechanisms employ a series of metal balls all in the same plane that are equally spaced around a circumference about the drive shaft. The balls are held between two circular plates each having an array of pockets to hold the balls. The spacing between the plates is therefore the ball diameter less the depth of the opposing ball pockets. A cage between the plates having a thickness slightly less than the plate spacing is usually employed to maintain even angular ball spacing. The plates and balls are held on the drive shaft by relatively heavy axial spring loading. Under normal operation, all parts rotate together at a commanded speed. The magnitude of the spring loading, the size and number of balls, and depth and shape of pocket dictate the torque limit of the device.
The breakout load or torque limit is selected to be greater than the maximum operating load so that it never “trips” during normal operation, but less than loads that would cause damage to the EMA. With the conventional ball detent mechanism described above, after a breakout or hard stop condition is encountered, one plate is brought to an abrupt stop while the other continues to rotate as the set of balls, in unison due to the cage, roll out of the pockets and onto the flat opposing surfaces of the two circular plates. The shaft is usually rotating at least several hundred—and often up to several thousand—revolutions per minute. The control electronics cannot sense a problem or act on a problem instantaneously, so the EMA's motor is driven for some fraction of a second after breakout. For example, if initial speed is 2400 RPM and six balls are used, with an assumed time of 200 msec before the motor can be turned OFF, 8 revolutions occur. Therefore, the balls that breakout of the initial pockets then encounter 48 more events of rolling into and out of subsequent pockets in the direction of rotation. With the high spring force and the abrupt shape of the pockets, the continued motion of the balls rolling into and out of pockets results in a very violent series of events. The balls experience very high and repeated impact loading and may fracture. Also, the edges of the pockets in the plates may generate harmful debris. Tests have shown significant damage to ball pockets after several encounters. The audible noise from the conventional approach is a loud chatter that may be described as “machine-gun-like.”